In recent years, an electronic apparatus typified by a mobile phone operates using electric power supplied from an external power source such as a battery or an AC (alternating current) adapter. Upon detecting an abnormal state in the electronic apparatus, a protection device of such an electronic apparatus generally shuts off supply of electric power from the external power source.
Such conventional protection device is disclosed in Patent Document 1, for example. FIG. 12 generally shows a protection device disclosed in Patent Document 1. The protection device according to Patent Document 1 uses a lithium-ion battery 80 as an external power source and includes an NMOS (n-type metal-oxide-semiconductor) transistor 81 for controlling a discharging current of the lithium-ion battery 80 and an NMOS transistor 82 for controlling a charging current of the lithium-ion battery 80.
In order to control the discharging current from the lithium-ion battery 80, a control circuit 83 outputs a signal V1 for controlling the NMOS transistor 81. In order to control the charging current to the lithium-ion battery 80, the control circuit 83 outputs a signal V2 for controlling the NMOS transistor 82. A temperature detection circuit 84 detects an ambient temperature of the NMOS transistor 81, and a temperature detection circuit 85 detects an ambient temperature of the NMOS transistor 82. The temperature detection circuits 84 and 85 output “H (high)” level signals, when the detected temperature is higher than or equal to a predetermined value, for example, 100° C.
When an output from the temperature detection circuit 84 is at a “L (low)” level, an NMOS transistor 86 connected between the gate and the source of the NMOS transistor 81 is in the OFF state, allowing the signal V1 to be applied to the gate of the NMOS transistor 81. Meanwhile, when the output from the temperature detection circuit 84 is at a “H” level, the NMOS transistor 86 is in the ON state, grounding the gate of the NMOS transistor 81 to turn OFF the NMOS transistor 81. That is, the ambient temperature of the NMOS transistor 81 exceeding a predetermined value is determined to be an abnormal state, and thus the discharging current from the lithium-ion battery 80 is shut off.
When an output from the temperature detection circuit 85 is at a “L” level, an NMOS transistor 87 connected between the gate and the source of the NMOS transistor 82 is in the OFF state, allowing the signal V2 to be applied to the gate of the NMOS transistor 82. Meanwhile, when the output from the temperature detection circuit 85 is at a “H” level, the NMOS transistor 87 is in the ON state, grounding the gate of the NMOS transistor 82 to turn OFF the NMOS transistor 82. That is, the ambient temperature of the NMOS transistor 82 exceeding a predetermined value is determined to be an abnormal state, and thus the charging current to the lithium-ion battery 80 is shut off.
While the NMOS transistor 81 and the NMOS transistor 82 are in the ON state, a voltage of the lithium-ion battery 80 is supplied via power source terminals 88 and 89 to the electronic apparatus.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 11-289656